The present invention relates to filling level measurement in industrial and non-industrial processes. More specifically, the present invention relates to measurement of product level height in a storage tank using an electromagnetic wave level gauge.
Preferred instrumentation for the measurement of product level (liquid or solid products) in storage vessels is based on non-contact techniques. One highly preferred non-contact technology is based on the use of microwaves. The basic principle involves transmitting microwaves towards the product surface and receiving reflected microwaves from said surface. The reflected microwaves are analyzed to determine the distance that they have traveled. Knowledge of the distance traveled and storage vessel height allows the determination of product level. Since it is known that the group velocity of electromagnetic waves equals the speed of light in vacuum divided by the square root of the dielectric constant of the corresponding medium in which the wave propagates, the distance that an electromagnetic wave travels can be determined, if the time of travel and the dielectric constant of the medium are known. The time of travel can be determined by measuring the phase shift of the reflected wave in relation to the transmitted wave. Further, the time of travel can be measured by using well-known radar techniques, such as pulse radar or frequency modulated continuous wave (FMCW)-radar.
Prior art microwave level transmitters in the process control industry often work at a frequency of about five to six GHz. They have fairly big antennas and thus need a big orifice in the storage vessel. Generally, an orifice of a diameter of about six inches or more is required for the antenna to be put in. However, a wide orifice in the storage vessel and a large antenna are sources of potential problems, since on the one hand, it is difficult to seal a big orifice tightly, and on the other hand, the transmission of electromagnetic waves by large antennas in storage vessels suffers from the formation of condensate. Also for many storage vessels it is not favorable, or even possible, to create an orifice of about six inches or more.
Therefore, it would be desirable to have a level transmitter small enough, so that the size of the antenna insertion orifice in the storage vessel could be decreased to 1.5 inches or less, particularly to 1 inch or less, or even down to 3/4 inch or 1/2 inch. Such a level transmitter thus needs a small enough antenna, which can fit through the small orifice in the storage vessel.
One solution would be to use a rod antenna. However, the use of rod antennae in storage vessels often has the disadvantage of reduced signal quality. Many containers and storage vessels have insertion orifices provided with more or less extended flanges. Small orifices generally have correspondingly low diameter flanges, through which the rod antenna extends. Problems arise from lateral emission of radiation, which can lead to false echos when the antenna is mounted inside a narrow flange (FIG. 3a). A solution has been described in German Patent DE 196 41 036. A metal casing is provided to surround the rod antenna within the flange (FIG. 3b). However, this prior art solution may cause other problems, through the extended protrusion, of the rod antenna into the vessel, which might cause it to get partly overflown by the product, if a certain filling level is exceeded. In that case, the determination of the filling level might not be as accurate as it was before. Additionally, contact with the vessel contents might cause the forming of condensate on the rod antenna, causing deteriorating performance.
It is not generally possible, however, to use a horn antenna small enough to be insertable in container orifices less than, say, 4 inches, with prior art transmitters, because the antenna can then not transmit 5-6 GHz signals at sufficient quality.
It has now been found that these problems can be solved by combining an antenna small enough to fit into a container orifice with a diameter of less than 4 inches, and often down to less than 1.5 inches, with a transmitter operating at a higher frequency than 6 GHz, and up to 20 GHz and higher.
It has been found to be highly favorable to increase the frequency of the electromagnetic waves used for level measuring, since this not only allows a decrease in the size of the antenna and thus of the orifice in the storage vessel, but also to use a horn antenna with small orifices, which creates less false echos when mounted inside a narrow nozzle and gets less disturbed by artifact signals and condensate, instead of using a rod antenna.
Another kind of antenna which could be used then is the so called microstrip antenna or patch antenna It consists of an etched conducting structure sitting on a typically round-shaped substrate, as is well known in the semiconductor industry. Since substrate and conducting structure are negligibly thin and the diameter of such an antenna gets very small if the frequency is well above 5.8 GHz, the volume of the antenna is comparatively small. Because of its small volume, it has been found to be highly favorable to use a microstrip antenna if the flange of the storage tank is too short for a hom antenna to fit into it, or if there isn't any flange at all.
By working with a microwave frequency of more than 5.8 GHz, particularly of more than 10 GHz, preferably more than 20 GHz, and particularly preferred of 24 GHz or more, it is possible to decrease the diameter of container orifice and horn antenna to 1.5 inches or less, particularly 1 inch or less, down to 3/4 inch or 1/2 inch, which permits using a threading to seal the level transmitter tightly to the storage vessel, instead of using a flange.
Threadings the size of e.g., about an inch in diameter are easy to make and seal. They are thus preferred, as compared to flange seals required for wide orifices.
For high accuracy in filling level determination, it is crucial for the signal to have a broad absolute bandwidth. Since the absolute signal bandwidth of an antenna is increased by increasing the average driving frequency, an increase of frequency will lead to a more accurate determination of the pathlength the signal has to travel and therefore to a more accurate determination of the filling level in the storage vessel.